Light

How white light splits into a spectrum, colour mixing, and how filters and objects appear coloured.

The electromagnetic spectrum is the range of all types of electromagnetic radiation, ordered by wavelength and frequency. It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All EM waves travel at the speed of light in a vacuum.

When light hits a smooth surface, it reflects so that the angle of incidence equals the angle of reflection. Both angles are measured from the normal to the surface.

Lenses refract (bend) light by using curved transparent surfaces. Convex (converging) lenses focus parallel light rays to a focal point. Concave (diverging) lenses spread light rays outward. Lenses are used in glasses, cameras, microscopes, and telescopes.

Mirrors reflect light according to the law of reflection — the angle of incidence equals the angle of reflection. Plane mirrors produce virtual, upright images the same size as the object. Concave mirrors converge light and can magnify; convex mirrors diverge light and give a wider field of view.

Refraction is the bending of light as it passes from one medium to another. The bending occurs because light travels at different speeds in different materials.

When light travels from a denser medium to a less dense medium at an angle greater than the critical angle, it is completely reflected back — this is total internal reflection (TIR). TIR is the principle behind optical fibres used in telecommunications and medicine.

Sound is a longitudinal wave that requires a medium to travel. Pitch depends on frequency — higher frequency means higher pitch. Loudness depends on amplitude — greater amplitude means louder sound. Sound travels faster in solids and liquids than in gases.

All waves share common properties. Wavelength is the length of one complete wave cycle, frequency is the number of waves per second, and amplitude is the maximum displacement from rest. These properties are related by the wave speed equation v = fλ.